The present invention relates to a glazing provided with an electric circuit including an electrically conducting substrate and a terminal for making electrical connection thereto. In particular, the circuit may be a heating element for defrosting or demisting the glazing, or an antenna circuit. The glazing may be for a vehicle window, or a window for an appliance or a building; in the case of a vehicle it may be a windscreen or rear window.
Various techniques are known for making electrical connection to a terminal. For instance, it is known from GB-A-2 223 385 to solder a terminal to a busbar or current collector. Alternatively, EP-A-410 766 discloses an electrical device comprising a substrate supporting a film-type heater track formed of conductive polymer ink. A connector member is bonded to the substrate by means of adhesive, and the heater track overlies at least an end portion of the connector member to provide an electrical contact between them.
It is also known from EP 278 611 A1 to employ an electrically conductive adhesive to bond a busbar onto a conductive layer formed on a substrate, which may be used as a window. Furthermore, in GB 1 393 887, the use of steel filings to make electrical contact through an adhesive coating is disclosed in the context of a rear window heating circuit.
However, FR 2 519 477 teaches that electrically conductive adhesives have various disadvantages, for example, they are not sufficiently durable under the conditions to which a vehicle window is typically subjected, and they are expensive. Instead, FR 2 519 477 suggests that it is preferable to employ a non-conductive adhesive to maintain a suitably configured terminal in intimate contact with an electrically conducting substrate so as to ensure electrical continuity.
Unfortunately, the invention of FR 2 519 477 is also not without disadvantages; it has been found that the inevitable mismatch of the thermal expansion coefficients of glass, metal and adhesive results in a weakening of the adhesive bond to the extent that electrical contact is adversely affected. Consequently, soldering remains a widely used technique for attaching electrical terminals to glazings, although the strength of the bonds produced is not always satisfactory, and there are increasing environmental concerns about metals, such as lead, which are commonly used in solders. Furthermore, the actual soldering operation is inclined to produce thermal stresses in the glass, which sometimes lead to damage.
It would be desirable to improve the attachment of electrical terminals to glazings, avoiding the disadvantages described above.
According to the present invention there is provided a glazing with an electric circuit including an electrically conducting substrate and a terminal for making electrical connection thereto, wherein the terminal is attached to the substrate by adhesive, characterised in that the electrical connection between the terminal and the substrate is provided by means other than, or additional to, physical contact between the terminal and the substrate.
The provision of a secure and reliable electrical connection between the terminal and the substrate allows adhesives to replace solder without attendant problems of electrical continuity. Modern adhesives are able to offer many advantages over solder. They may be flexible in the cured or set condition, and thereby absorb stresses generated in, or exerted on, the terminal. Adhesives are available which do not require to be heated in order to form a bond, or only need to be heated to a modest temperature, and yet which still provide a strong bond. Furthermore, it is usually possible to arrange the manufacturing operation so that the adhesive is heated just before it contacts the glazing, thereby avoiding the need to heat the glazing directly. Durable electrically conducting adhesives are now available, thereby allowing the adhesive to fulfil both the mechanical and electrical functions of a conventional soldered joint. Generally, such adhesives contain finely dispersed metal powder to make them conductive. Providing that the electric current to be carried is not large, the adhesive constitutes a sufficient electrical connection. A further important advantage is that suitable adhesives are available which have fewer health and safety hazards and less adverse environmental impact than the conventionally used solders.
Preferably the terminal comprises separate base and connector parts which are adapted to mutually engage, and the base part of the terminal is attached to the glazing by adhesive.
Advantageously, especially in the automotive industry, the base may be of a standardised design of general applicability, whereas the connector part may be tailored to the specific requirements of a particular glazing, vehicle or vehicle manufacturer. That is, the base is preferably usable with a variety of differing designs of connector part.
Means of electrical connection between the terminal and the substrate may, for example, include the use of an electrically conducting adhesive, and/or a soldered connection. In high current applications, a joint soldered directly between the terminal and the substrate is preferable as the additional means of electrical connection, in order to obtain a low resistance connection. In this situation the solder complements the adhesive, as the latter bears the mechanical load and stresses, leaving the solder to provide, or enhance, electrical continuity without being compromised. This makes it possible, for instance, to use solders which are less environmentally aggressive, even if there is some loss in bond strength.
The invention also provides a method of making a glazing with an electric circuit including an electrically conducting substrate and a terminal for making electrical connection thereto, comprising the steps of:
providing a pane carrying the electric circuit and substrate, and
attaching the terminal to the substrate with adhesive, characterised by
providing the electrical connection between the terminal and the substrate by means other than or additional to, physical contact between the terminal and the substrate.
Improvements can also be made in the type and method of application of the adhesive.
Preferably the adhesive is provided in the form of a tack-free tablet which is placed in contact with the terminal. The use of a tack-free (i.e. non-sticking) form of adhesive is advantageous because handling of the adhesive is greatly facilitated.
It is also preferable for the adhesive to be pre-shaped to match the part of the terminal that the tablet is in contact with. This increases the area of the terminal that is bonded, and hence the strength of the bond, while reducing the likelihood of adhesive escaping from underneath the terminal and becoming visible, which is likely to be unsightly. It also results in less waste and hence a cost reduction.
Advantageously the adhesive is pre-applied to the terminal, e.g. the terminals may be supplied with adhesive already applied to the requisite part of the terminal.
Optionally the adhesive may be heated to activate or cure it. The term xe2x80x9cactivatingxe2x80x9d an adhesive is used to refer to any process which initiates the bonding process, e.g. one which makes a previously tack-free adhesive sticky, or one which starts the curing process. Activation may involve melting the adhesive (at least on its surface), initiating a heat-dependent chemical reaction, or removing or destroying a barrier which separates two reactants. The extent of heating an adhesive is frequently less than is required for solder, and with regard to heating for activation, the adhesive need not necessarily be in contact with the glazing at the time of heating.
In this specification, the ""term xe2x80x9csolderxe2x80x9d is used to denote a fusible alloy of metals. The term xe2x80x9cadhesivexe2x80x9d is used to denote a substance employed to bond other substances together, but does not include a solder as such. An adhesive may be wholly non-metallic or may contain a metallic component.